This invention relates generally to instruments used to measure or detect a condition of the fetus in utero and, in particular, to pulse oximeter sensors used to measure the blood oxygen saturation of the fetus during labor and delivery.
Pulse oximeters are typically used to measure various blood characteristics, including arterial blood oxygen saturation and pulse rate. Pulse oximetry sensors pass light through a portion of the patient's tissue and photoelectrically detect pulsatile changes in the absorption of the light by the tissue. The detected light is then used to determine the characteristic of interest.
Pulse oximetry sensors generally fall into two categories. Transmissive pulse oximetry sensors shine light through opposed blood perfused tissue surfaces, such as a finger or an ear, by disposing the light emitters and photodetectors on opposite sides of the tissue. Transflectance sensors, on the other hand, emit light into and detect light from the same side of the tissue.
The quality of the optical signal generated by the pulse oximeter sensor depends on the quality of optical coupling between the sensor and the patient. Optical coupling refers to a relationship between two objects permitting light to be transmitted from one object to the other. In the context of a pulse oximeter sensor and a patient, optical coupling refers to a relationship between the sensor and the patient permitting the sensor to transmit light into the patient's blood-perfused tissue and permitting a portion of the light to return to the sensor after passing through the tissue. The quality of the optical coupling is related to the amount of light emitted by the sensor that actually enters the patient's tissue and to the portion of the light received by the sensor that actually passed through the patient's blood-perfused tissue.
Tissue characteristics at the sensor site can affect the quality of the optical coupling between the sensor and the patient. For example, the presence of hair or mucous on the skin will attenuate the light transmitted into the tissue by the sensor.
In addition, the physical position and orientation of a sensor with respect to the patient's skin will affect the optical coupling of the sensor with the patient. An improperly applied sensor may permit some of the light from the emitters to shunt directly to the photodetector without passing through the patient's tissue. This latter problem is more prevalent with transflectance sensors than with transmissive sensors.
Pulse oximeters may be used to measure fetal blood oxygen saturation during labor and delivery. Since the accessible part of the fetus (usually the top of the head) does not offer opposed tissue surfaces for transmissive pulse oximetry, transflectance sensors are used. The use of transflectance sensors in the fetal environment presents some unique optical coupling problems, both as to tissue characteristics at the sensor site and as to retention of the sensor at the chosen site.
Prior art fetal pulse oximetry sensors were placed on the portion of the fetus showing through the dilated cervix (the "presenting part") or on the portion of the fetus within the uterus and adjacent to the cervix (the "transcervical region"). Sensors placed on the presenting part were typically attached by hooks inserted through the fetus' skin or by suction to retain the sensor in place. Sensors placed on the transcervical region were held in place by the pressure of the cervical wall against the fetus. While neither fetal tissue region could be seen by the user, both regions could be reached by the user's fingers to ensure that the sensor was firmly in place on the fetus to provide adequate optical coupling between the sensor and the tissue.